Aqueous ink for inkjet recording generally is used in the form of a solution obtained by dissolving a water-soluble dye in water or an aqueous solution containing a water-soluble organic solvent, or in the form of a dispersion obtained by dispersing a water-insoluble color material into water or an aqueous solution containing a water-soluble organic solvent. The ink for inkjet textile printing using a disperse dye is used for a method, for example, in which a fabric made of a hydrophobic fiber material is subjected to inkjet printing and is then heated to be dyed. Examples of the characteristics required for ink for inkjet recording include the following:
(1) ejection stability while preventing nozzle clogging;
(2) high-quality recorded images resulting from uniformity in the direction, the ejection amount, and the dot shape of ink ejected;
(3) ink storage stability;
(4) high print density and high image quality; and
(5) quick drying and fixation.
In an inkjet recording system, it is required to eject ink from thin nozzles as ink droplets, and therefore, item (1) of the above-described items is particularly important. In particular, in an on-demand type inkjet recording system, there is a problem in that the ejection of ink from the nozzles is temporarily stopped even during a continuous operation, and thus an ejection failure occurs during long continuous operation. Unlike ink for inkjet textile printing using a water-soluble dye, ink for inkjet textile printing in the form of a dispersion using a disperse dye is likely to cause clogging because precipitates (aggregates) are generated due to the degradation of dispersibility and the viscosity increases due to the evaporation of moisture in the nozzles, for example. In particular, when the device is not operated for a long time, clogging is likely to occur due to the evaporation of moisture. Therefore, a humectant is added to suppress the evaporation of moisture. Examples of the humectant include a high-boiling water-soluble organic solvent and a solid water-soluble compound. Polyhydric alcohol-based compounds such as glycerin and polyethylene glycol, pyrrolidone-based compounds such as 2-pyrrolidone and N-methyl-2-pyrrolidone, and the like are used as the high-boiling water-soluble organic solvent, and sugars, urea-based compounds and the like are used as the solid water-soluble compound.
Patent Document 1 describes ink for inkjet recording characterized by containing a water-insoluble color material, a dispersant, and a polyoxyethylene alkyl ether-based compound with an alkyl group having 25 to 150 carbon atoms. Since the dispersant and the polyoxyethylene alkyl ether-based compound with an alkyl group having 25 to 150 carbon atoms are mixed together in the ink for inkjet recording described in Patent Document 1 in order to improve a redispersion property, the surface tension of the ink tends to be low. Therefore, when inkjet printing using that ink is performed on a fabric made of a hydrophobic fiber material, the ink sometimes exhibits excessive penetration into the fabric, and thus the density of the ink on the surface of the fabric tends to be low.
Patent Document 2 describes an aqueous ink composition in which a water-insoluble coloring agent has an average dispersed particle size of 0.1 to 0.3 μm, a styrene-acrylic acid-based copolymer has an acid value of no less than 150 mg KOH/g to less than 250 mg KOH/g as a free acid, and a proportion of the styrene-acrylic acid-based copolymer with respect to the water-insoluble coloring agent is no less than 5 mass % to less than 20 mass %. Patent Document 2 states that the water-insoluble coloring agent has an average dispersed particle size within a specific range of 0.1 to 0.3 μm, which is relatively large, and thus it is possible to achieve an extremely high recording density. However, since the proportion of the styrene-acrylic acid-based copolymer with respect to the water-insoluble coloring agent is no less than 5 mass % to less than 20 mass %, a sufficient dispersion stability (ink storage stability) of the water-insoluble coloring agent is not attained.
Patent Document 3 describes an ink composition containing an acetylene glycol compound as a penetrant and a styrene-(meth)acrylic acid-based water-soluble resin as a water-soluble dispersant in ink containing a specific pigment. Patent Document 3 states that the ink composition containing a specific pigment together with specific components achieves images having an excellent rubbing resistance and color reproducibility, and has advantages as an extremely permeable ink composition. The styrene-(meth)acrylic acid-based water-soluble resin described in each of the examples in Patent Document 3 has a weight-average molecular weight of only 7,000 and an acid value of 70 to 150, and the acetylene glycol compound is unspecified. In the examples, only “Surfynol 465” is described as the acetylene glycol compound.
Patent Document 4 describes disperse dye-containing inkjet ink that contains a polymer resin having a carboxyl group or a sulfonic group as an acidic group with an acid value of no less than 80 mg KOH/g to no more than 300 mg KOH/g in an amount of no less than 2 mass % to no more than 10 mass % as a solid content, the polymer resin being used not as a dispersant but as a binder resin, the inkjet ink being capable of achieving high-quality and highly durable images only by heating a fabric.